1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus such as an electrophotographic apparatus, and more particularly to a developing device for developing an electrostatic latent image with a dry-type one-component developer.
2. Related Background Art
In dry-type one-component developing devices, it is preferable to provide a developer removal/supply member to remove a developer from the surface of a developer bearing member passing through a developing portion at a position upstream of a regulating member for regulating the thickness of a developer layer born by the developer bearings member to the developing portion with respect to a rotational direction of the developer bearing member, as well as supplying the developer onto the surface of the developer bearing member proceeding toward the regulating member.
A well known developer removal/supply member is a brush roller or foamed rubber roller which rotates in contact with a developer bearing member within a developer containing vessel, as described in U.S. Pat. Nos. 5,075,728 and 5,086,728. The foamed rubber roller may be a continuously-porous or independently-porous foamed rubber roller.
However, a resilient roller of the brush structure as above described is constructed by flocking fibers such as nylon or rayon on a metallic core, wherein there may be clogging between the regulating member and the developer bearing member due to fibers falling out from repetitive use of the developing device over a long term. As a result, there were some cases where white streaks arose on defective portions, and the resilient roller failed to make contact with the developer bearing member because of deterioration of the fibers, whereby the reproduction of a full black image was sometimes insufficient due to ineffective supply and peeling off of developer. Thus, the resilient roller was constituted of a continuously-porous cellular material (with cells communicating with each other, such as polyurethane having a relatively low hardness and placed into contact with the developer bearing member at low pressure, without applying any excessive pressure on the developer, whereby it was made possible to supply the developer onto the developer bearing material and peel off the developer left unconsumed by the development, owing to adequate irregularities on the surface of cellular material.
However, in such a developing device, particularly when a fine grain developer is used under high humidity environments, the developer may gradually enter into the depth of the resilient roller in a number of repetitive developing operations, because the resilient roller is formed of a continuously-porous cellular material having a relatively low density. If such interstitial developer spread over the entire area of the resilient roller, the resilient roller became rigid, and the contact pressure against the developer bearing member became excessive, increasing the driving torque for the developer bearing member and the resilient roller, or causing some unevenness in the developer on the developer bearing member which could be applied and peeled off by the resilient roller, so that the thickness of a developer layer on the developer bearing member might undesirably vary.
In order to prevent the inconvenience due to clogging of developer within the resilient roller, the following constitution can be considered.
1 A non-cellular skin layer for preventing the penetration of developer is provided on the surface of the resilient roller (U.S. Pat. No. 5,086,728).
2 The resilient roller is constituted of an independently-porous cellular material (cells not communicating with each other).
3 Cells (bubbles) of a continuously-porous cellular material are made as fine as possible to obtain a higher density roller.
However, when such a constitution is used for the resilient roller, the following inconveniences may arise. First, with the constitution 1, if the skin layer is placed into contact with the surface of the developer bearing member and rubbed, the action of rubbing the developer on the surface of the skin layer against the developer bearing member may be too strong, and when a number of developing operations are made, the developer may be fused onto the developer bearing member, or the developer material may be changed (developer deterioration), so that the fog might undesirably increase. Also, if the surface of the skin layer is made adequately coarse, the hardness becomes locally higher as compared with the surface of the cellular material (with the local pressure against the developer bearing member being higher), whereby the same problem could not be prevented.
Next, the constituion 2 was described in U.S. Pat. Nos. 5,075,728 and 5,086,728, but no disclosure of the problems and their measures with the independently-porous cellular material roller was made.
With the constitution 3, if a continuously-porous cellular material having cells as fine as possible (a cell number of 100 cells/inch) is used, rather than the continuously-porous cellular material having the typical number of cells, for example, a cellular material such as polyurethane foam having the cell number of 30 to 50 cells/inch, the entry of toner into the inside of the resilient roller can be prevented to some extent, but the overall hardness of the resilient roller becomes too high. Hence, if such an arrangement is taken that the resilient roller is stably and securely placed into contact with the developer bearing member, the contact pressure against the developer bearing member as well as the developer becomes excessive, increasing the driving torque for the developer bearing member and the resilient roller or raising the fog associated with the developer material change (toner deterioration) due to durability, whereby there occurs some inconvenience.